In synchronous motors that use permanent magnets, and in particular in three-phase synchronous motors in which the stator windings are concentratedly wound around the teeth, the ratio between the number of magnetic poles of the permanent magnets used in the rotor and the number of slots (i.e., the number of teeth) of the stator is often 2:3.
In a synchronous motor in which the ratio between the number of magnetic poles and the number of slots of the stator is 2:3, openings are often provided between respective adjacent teeth. The openings are provided in order to facilitate interlinkage of the magnetic fluxes generated by the permanent magnets disposed in the rotor with the stator windings and to prevent the magnetic fluxes generated because of the electric current flowing in the stator windings from short-circuiting between the teeth of the stator without the generated magnetic fluxes being directed to the rotor.
However, near the openings, because the magnetic flux density distribution in the gap between the stator and the rotor is disturbed, cogging torque is generated, and the cogging torque causes vibration and noise.
In order to reduce such cogging torque, conventional synchronous motors, a representative example of which is disclosed in Patent Literature 1 listed below, are configured such that a rotor having 8 or 10 magnetic poles and a stator having 9 slots are used and the windings for one phase are concentratedly wound around three adjacent teeth of each phase.
In such synchronous motors, because 9 windings are disposed on the stator, the teeth are disposed at intervals of a mechanical angle of 40 degrees and the windings on the teeth are continuously disposed at intervals of a mechanical angle of 40 degrees. In a rotor having 8 poles, the width of one magnetic pole has a mechanical angle of 45 degrees. In a rotor having 10 poles, the width of one magnetic pole has a mechanical angle of 36 degrees.
Further, pulsations generated while the rotor rotates 360 degrees are determined by the least common multiple of the number of slots of the stator and the number of poles of the rotor. Thus, in the case where a synchronous motor having a ratio between the number of magnetic poles and the number of slots of the stator of 2:3 is, for example, an 8-pole/12-slot synchronous motor, pulsations occur 24 times. However, in the case of an 8-pole/9-slot synchronous motor, pulsations occur 72 times, and in the case of a 10-pole/9-slot synchronous motor, pulsations occur 90 times.
As the number of pulsations increases, the energy of the cogging torque becomes more distributed. Thus, the amplitude of the cogging torque decreases. In other words, an 8-pole/9-slot or 10-pole/9-slot synchronous motor can reduce the cogging torque more than a synchronous motor that has a ratio between the number of magnetic poles and the number of slots of the stator of 2:3.